In commercial inkjet printing systems, a print media is physically transported through the printing system at a high rate of speed. For example, the print media can travel 650-1000 feet per minute. A commercial inkjet printing system typically includes multiple lineheads that each jet ink or another type of liquid onto the moving print media. The width of a print media can range from 8.5 inches to 52 inches. To ensure ink can be deposited across the different widths of the print media, each linehead typically includes multiple printheads arranged to cover the varying widths of different types of print media.
FIG. 1 illustrates a side of a linehead that is adjacent to a print media. The linehead 100 includes six printheads 102 in the illustrated embodiment. The printheads 102 are aligned in a staggered formation, with upstream and downstream printheads 102, such that the nozzle arrays 104 produce overlap regions 106. Each nozzle array 104 includes one or more lines of openings or nozzles that emit ink drops. The number of nozzles in a printhead can number in the thousands while the number of nozzles in an overlap region can number less than one hundred.
FIGS. 2 and 3 depict a portion of two printheads 102 in an overlap region 106. Each nozzle array 104 includes a single line of nozzles, and ten nozzles 200 are included within the overlap region 106 in the illustrated embodiment. The overlap regions 106 enable the print from overlapped printheads 200 to be stitched together without a visible seam through the use of appropriate stitching algorithms that are known in the art. For example, U.S. Pat. No. 7,871,145 and United States Patent Application Publication 2011/0012949 disclose methods for printing that reduce stitching errors or artifacts.
Typically, each nozzle can be independently controlled to jet ink or to not jet ink. A stitching algorithm is used to determine the best combination of nozzles to jet ink in the overlap regions to reduce or eliminate stitching artifacts. Stitching artifacts are produced when a stitch boundary in the overlap region is over-printed or under-printed. For example, a stitching algorithm can determine nozzle 202 (indicated by “X”) and nozzle 204 (indicated by “●”) alternately jet ink drops. The stitch boundary is the area 206. The ink dots 208 produced on the print media illustrate the alternating jetting of ink drops from the nozzles 202, 204. The actual placement of the ink dots 208 on the print media has been determined by an operator to reduce the occurrence of darker or lighter lines (e.g., stitch artifacts) in the printed content.
Alternatively, as shown in FIG. 3, a stitching algorithm can determine nozzles 300, 302, 304, 306, 308 jet inks in varying groups (indicated by “X”). The nozzles 310, 312, 314, 326, 318 jet ink in varying groups (indicated by “●”). The stitch boundary, indicated by 320, corresponds to the overlap region in the illustrated embodiment. The ink drops jetted from the nozzles produce a sawtooth or “s” pattern of ink dots on the print media. For example, when nozzles 300, 302, 304, 306, 308 jet ink drops, ink dots 322 are produced on the print media. When nozzles 300, 302, 310, 312, 314 jet ink drops, ink dots 324 are produced on the print media. And when nozzles 310, 312, 314, 316, 318 jet ink drops, ink dots 326 are produced on the print media. Stitching algorithms ensure the amount of ink printed in the stitch boundary 320 in the overlap region 106 is not higher or lower than other areas of the printed content.
FIGS. 4-6 illustrate stitched printed content in an overlap region. FIG. 4 depicts printed content 400 that does not include any stitching artifacts. The proper combination of nozzles to jet ink in the overlap region has been determined such that the ink coverage in the overlap region has limited overlap and a minimal amount of unprinted areas.
FIG. 5 illustrates printed content 500 that includes a stitching artifact 502 produced by over-printing in or around the stitch boundary in the overlap region. Too many nozzles jetted ink and produced the dark band artifact 502. FIG. 6 depicts printed content 600 that includes a stitching artifact 602 produced by under-printing in the overlap region. Too few nozzles jetted ink and produced the light band artifact 602. The artifacts 502 and 602 can become visible when the size of the dark or light band is sufficiently large.
Stitching artifacts continue in the direction the print media is traveling until the stitching algorithm is adjusted. Unfortunately, the necessary corrections may not occur for hundreds or thousands of feet of print media, which results in waste when the printed content is not usable. Additionally, wasted print media causes the print job to be more costly and time consuming.
Stitching artifacts can range in size from microns to millimeters. Current stitching artifact detection systems use high resolution and high magnification cameras designed to detect discrete artifacts in the printed content. The high resolution and high magnification cameras are purposefully designed to avoid blur in the captured image so that an operator can see each ink dot deposited on the surface of the print media in an overlap region and determine the effectiveness of a stitching algorithm. Due to the expense of these high resolution and high magnification camera, only a few are typically used in a commercial printing system. In some printing systems, only two cameras are provided; one camera for each side of the print media. In such printing systems, a camera may have to be moved from one overlap region to another overlap region to determine the most effective stitching algorithm to use. This serial method of stitch correction takes time to accomplish and requires many pages to be printed during the correction process. More importantly, the high resolution and high magnification cameras are not configured to continuously monitor multiple overlap regions during a print job.